Explosives Detection attracts much attention in the field of social and public security. It is of special significance to explore quick and effective detection technologies to safeguard people's life and property security and construct harmonious society. As anti-terrorism intensity increases and as security check and anti-explosive measures are reinforced, the existing short-distance explosive detecting and identifying apparatuses are playing a substantial role. However, criminals continually enhance their counter-reconnaissance awareness and the explosive apparatus varies therewith, hazardous substance may explode during the inspection stage, thus threaten security of inspectors and detecting apparatuses, so it is the most optimal means to perform a long-distance detection.
Currently, the technologies meeting the demand for long-distance explosive detection to a certain extent mainly includes X-ray backscattering imaging, laser spectroscopy, thermal imaging, millimeter wave and terahertz technologies and the like (1. A study and Application of Long-distance Explosive Detection Technologies, Qianjing TANG and Jie SHAO, China Security & Protection, 2009, 9:40-45, which is incorporated herein by reference in its entirety). The X-ray backscattering imaging technology uses the back-scattered X-rays for imaging the detected object, wherein the X-ray energy used therefor is lower than the energy used for transmission imaging, and the potential detection distance for the X-ray backscattering imaging technology is 15 meters, which is sufficient for distinguishing the explosives from the background. Since the X-rays have ionization-inducing property, they do harm to people's health to a certain degree. Laser spectroscopy judges whether there is an explosive mainly by taking advantage of the laser with a particular wavelength absorbed or emitted by the object being detected upon laser radiation, for example, Raman spectrum, laser induced fluorescence spectrum and photoacoustic spectrum. The laser spectroscopy technology is advantageous in that laser has a good propagation characteristic and meets the demand for long-distance detection, and this technology is disadvantageous in that laser cannot penetrate an opaque object and therefore cannot be used to detect hidden explosives. Thermal imaging technology performs detection mainly by means of the temperature difference between the hidden substance and the surface. This technology is remarkably advantageous for detecting body bombs, but flow of air and other thermal sources may have an influence on the detecting results. Meanwhile, this technology can only provide information about the shape of the hidden substance so that explosives cannot be discriminated from the perspective of the substance composition, therefore the detection capability of this kind of technology is limited. As to millimeter wave technology, images are formed by electromagnetic radiation of millimeter wave band emitted by the detected object itself or reflected back from the object. The millimeter wave has excellent penetrability through atmosphere and clothing and is capable of detecting hidden weapons at a long distance, but does not have an ability to identify substance composition. Terahertz radiation generally refers to electromagnetic waves with a frequency in a range of 0.1-10 THz and it has unique properties in the following aspects: first, a lot of organic molecules are characterized by characteristic absorption and dispersion within a terahertz frequency band so that the terahertz spectrum of a substance exhibits a “fingerprint” property. Hence, a substance species and composition can be identified by means of terahertz spectroscopy technology; secondly, terahertz radiation has a very strong penetration through many non-metallic and non-polarity substances and can directly detect hidden hazardous substance; besides, terahertz electromagnetic waves do not have ionization-inducing property as the X-rays and is not harmful to the materials and human body, so terahertz technology embraces an excellent application prospect in respect of explosive detection.
In 2006, US landforce RDECOM CERDEC Night Vision and Electronic Sensor Laboratory developed a set of 640 GHz active imaging instrument which can detect concealed weapons (2. E. L. Jacobs, S. Moyer, C. C. Franck, et al., Concealed Weapon Identification Using Terahertz Imaging Sensors. Proc. Of SPIE, 2006, 6212: 62120J, which is incorporated herein by reference in its entirety). Its detection distance is about 1.5m, the confocal imaging manner employed ensures high resolution and signal-to-noise ratio (SNR), but the scanning speed thereof is slow. At the same time, German Aerospace Research Center conducted a research of stand-off terahertz imaging for metal hazardous substances concealed below peoples' clothes for the anti-terrorism purpose, and in 2007 successfully developed an imaging system prototype machine with an operation frequency of 0.8 THz, a detection distance up to 20 m and a resolution of less than 2 cm (3. H.-W. Hü bers, A. D. Semonov, H. Richter, et al., Terahertz imaging system for stand-off detection of threats, Proc. Of SPIE, 2007, 6549:65490A, which is incorporated herein by reference in its entirety), which can reach a scanning speed approaching real-time image collection. The above research indicates that it is feasible to take advantage of active terahertz radiation for imaging and positioning a suspicious object at a long distance, but it needs to combine with spectrum information to identify with respect to the explosive detection. Furthermore, such research is still in a bench test stage and not put into practical application yet, which need to be further developed.
Few studies at home and abroad are carried out for long-distance terahertz spectroscopy and they are all in an exploration stage. In 2006, US RPI Terahertz Research Center adopted conventional terahertz time-domain spectroscopy technology to detect an explosive sample at a distal distance, observed RDX 0.82 THz absorption peak even at a propagation distance of 30 m and preliminarily found that long-distance explosive identification is feasible (4. H. Zhong, A. Redo, Y. Chen, et al., THz wave standoff detection of explosive materials, Proc. of SPIE, 2006, 6212: 62120L, which is incorporated herein by reference in its entirety). However, atmospheric absorption results in serious distortion of spectrum, undesirable signal-to-noise ratio and is inapplicable for practical application. This research center further proposed a new technology of generating pulsed terahertz radiation by inducing air plasma by means of femtosecond laser (5. J. Dai and X.-C. Zhang, Terahertz wave generation from gas plasma using a phase compensator with attosecond phase-control accuracy. Appl. Phys. Lett., 2009, 94: 021117, which is incorporated herein by reference in its entirety). As such, visible light in the atmosphere with good transmission property may be emitted to nearby the detected object at a distal distance to produce terahertz radiation so as to avoid attenuation caused by the atmosphere to the terahertz radiation, then the explosive is identified by spectroscopy. However, long-distance detection of a reflected signal is confronted with difficulty, pure spectroscopy technology only detects one measuring point of the object and does not have a spatial orientation capability. Therefore, this technology needs to combine with an imaging technology to meet demands of practical application.